EQUILIBRIUM OF A PARTICLE 21 (a) (d) SLIDING FRICTION. 26. Frictional Force.—Consider the forces acting upon a body which is in equilibrium on a rough inclined plane, Fig. 17. The body is acted upon by two i ^ forces, namely, its weight, W, and the reaction of the plane, R. The reaction of the plane is the result of two distinct and independent forces. One of these, N, is perpendicular to the plane and is called the normal reaction. The other, F, is along the plane and is called the frictional force. The normal reaction is due to the rigidity of the plane. It resists the tendency of the body to go through the plane. The frictional force is due to the roughness of the contact between the body and the plane. It prevents the body from sliding down the plane. 27. Angle of Friction. — As we increase the angle of elevation of the inclined plane a certain definite angle will be reached FIG. 17.